Accurate borehole dimension data are important for well logging and well completion. Measurements performed by many logging tools, whether wireline, logging-while-drilling (LWD), or measurement-while-drilling (MWD) tools, are sensitive to borehole sizes or tool standoffs. Therefore, accurate borehole dimension information may be required to correct measurements obtained with these tools. Furthermore, information regarding a borehole dimension is used to determine well completion requirements, such as the amount of cement required to fill the annulus of the well. In addition, borehole dimension data may be used to monitor possible borehole washout or impending borehole instability such that a driller may take remedial actions to prevent damage or loss of the borehole or drilling equipment.
Borehole dimensions, such as diameter, may be determined with various methods known in the art, including ultrasound pulse echo techniques disclosed by U.S. Pat. Nos. 4,661,933 and 4,665,511. Such ultrasound measurements rely on knowledge of the velocity of the ultrasound pulse in the particular medium, e.g., drilling fluids.
However, the velocity of an ultrasound pulse, typically, is not easily measured in a wellbore. Instead, the velocity of an ultrasound pulse in the well is typically extrapolated from an ultrasound velocity measurement made at the surface based on certain assumptions concerning the mud properties under downhole conditions. Such assumptions may not be accurate. Furthermore, mud properties in a drilling operation may change due to changes in the mud weight used by the driller, pump pressure, and mud flow rate. In addition, the drilling mud may become contaminated with formation fluids and/or earth cuttings. All these factors may render inaccurate the velocity of an ultrasound pulse estimated from a surface determination.
Therefore, there is a need for improved methods and apparatus for the measurement of ultrasound velocity in downhole environments.